1. Field of the Invention
The present invention relates to an optical transceiver and, more particularly, to encoding and clock signal extraction means for an optical code division multiplexing (OCDM) transceiver.
2. Description of Related Art
Metro-areas that are located between long-distance networks (also known as an ‘Internet backbone’) and access lines necessitate higher communication speeds and increased capacities. Although bit rates of Internet backbones have already been secured up to Terabit communication bandwidths, higher communication speeds and larger capacities for metro areas have fallen behind. Subsequently, when the background of the expansion of the Internet and advances made with wider bandwidths for content is considered, there is a demand for higher communication speeds and larger capacities in metro areas.
In order to afford communications a larger capacity, optical multiplexing technology that transmits a plurality of channels' worth of optical pulse signals all together over a single optical fiber transmission line has been studied. In optical multiplexing technology, optical time division multiplexing (OTDM), wavelength division multiplexing (WDM) and OCDM have been vigorously researched. Of these, OCDM possesses flexibility from an application standpoint, that is, superior characteristics such as that of not being restricted on the time axis allocated to each single bit of the optical pulse signal that is sent and received by OTDM and WDM.
OCDM communication is a communication method that extracts a signal by means of pattern matching by allocating different codes (patterns) to each channel. That is, OCDM is optical multiplexing technology that encodes, on the transmission side, an optical pulse signal by means of an optical code that is different for each communication channel and which, on the reception side, performs decoding to restore the original optical pulse signal by using an optical code that is the same as that of the transmission side for each communication channel.
During decoding, processing is performed to extract, as a valid signal, only an optical pulse signal in which code is mixed and, therefore, an optical pulse signal that comprises the same wavelengths or light rendered by combining the same wavelengths can be allocated to a plurality of communication channels. Further, an optical encoder is capable of using a passive optical element such as a Fiber Bragg Grating (FBG) and is therefore not subject to electrical restrictions, whereby adaptation for an increased signal rate is possible. Further, a plurality of channels can be multiplexed with the same wavelength at the same time, whereby high-capacity data communications are made possible. In comparison with OTDM and WDM, OCDM is noteworthy in that the communication capacity can increase very quickly.
Encoding and decoding methods include direct spreading methods, time spreading methods, wavelength hopping methods, and time spreading/wavelength hopping methods and so forth. Hereinafter, code that is used in time spreading/wavelength hopping methods is known as time spreading/wavelength hopping code. The present invention is an invention relating to OCDM that employs time spreading/wavelength hopping code (see Japanese Patent Application Laid Open No. 2000-209186, “Enhancement of transmission data rates in incoherent FO-CDMA systems”, X. Wang and K. T. Chan, OECC 2000, 14A2 to 5, p. 458, (2000), “Optical Code Division Multiplexing (OCDM) and Its Applications to Photonic Networks”, Ken-ichi Kitayama, Hideyuki Sotobayashi, and Naoya Wada, IEICE Trans. Fundamentals, Vol. E82-A, No. 12 (1999), and “Transparent Virtual Optical Code/Wavelength Path Network”, Hideyuki Sotobayashi, Wataru Chujo, and Ken-ichi Kitayama, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 8, No. 3 (2002), for example).
Therefore, the principles of encoding and decoding using time spreading/wavelength hopping code will first be explained with reference to FIGS. 1A to 1D, and FIGS. 2A to 2D.
FIG. 1A serves to illustrate encoding and decoding in a case where code set for an encoder and code set for a decoder are the same. Further, FIGS. 1B to 1D illustrate the process from the point where an optical pulse signal is encoded and transmitted until same is received and decoded. That is, the appearance of an optical pulse signal before and after encoding and before and after decoding is shown with respect to the time axis. Meanwhile, FIG. 2A serves to illustrate encoding and decoding in a case where code set for an encoder and code set for a decoder are different. Further, FIGS. 2B to 2D are the same as FIGS. 1B to 1D. In FIGS. 1B to 1D and 2B to 2D, the horizontal axis is a time axis that shows time by means of an optional scale and the vertical axis shows light intensity by means of an optional scale.
For the sake of expediency in the following description, an expression such as an optical pulse signal is used only in cases where an optical pulse train that reflects a binary digital electrical signal obtained by converting an electrical pulse signal to an optical pulse signal by light-modulating an optical pulse train is intended. Meanwhile, the expression of an optical pulse train is used to indicate the totality of lined up optical pulses over a fixed interval that is regular on a time axis (‘data cycle’ sometimes appears hereinafter). A data cycle is also generally called a ‘time slot’.
Furthermore, the bit rate, which expresses the communication speed, is the speed indicating whether it is possible to send and receive information on how many bits per unit time and is the reciprocal of the data cycle. Further, the maximum spread time per bit (one optical pulse) is also known as the code cycle. That is, the code cycle is the maximum time width that is allocated to each optical pulse constituting an optical pulse signal on the time axis of an encoded optical pulse signal. In FIGS. 1B to 1D and 2B to 2D, the data cycle is indicated by Tb and the code cycle is indicated by Tc.
In FIGS. 1B to 1D and 2B to 2D, the optical pulses are schematically shown as follows. That is, the optical pulses that constitute the optical pulse signal are not actually square waves as illustrated but are shown as square waves for the sake of expediency. Further, in FIGS. 1B to 1D and 2B to 2D, the optical pulses constituting the optical pulse signals are shown as a mixture of light of three different wavelengths (λ1, λ2, and λ3). In order to indicate that this is light of the wavelengths λ1, λ2, and λ3, the references λ1, λ2, and λ3 are appended to the rectangles representing the optical pulses.
For example, in FIG. 1B, the optical pulses constituting the optical pulse signals are generated from light rendered by mixing the wavelengths λ1, λ2, and λ3. Therefore, the rectangles to which the codes of λ1, λ2, and λ3 have been appended are shown stacked on the time axis. The same is true for FIGS. 1D and 2B. On the other hand, in FIGS. 1C and 2C, an optical pulse signal is encoded and the optical pulses are spread over the time axis. Therefore, optical pulses that are shared for each unit data cycle are arranged divided into optical pulses (also known as ‘chip pulse’ hereinafter) with a single wavelength for each of the wavelengths λ1, λ2, and λ3.
In order to represent this situation, FIGS. 1C and 2C show chip pulses by means of rectangles to which codes such as λ1 and λ2 are appended in correspondence with the wavelength of each chip pulse. As will be described subsequently, in the OCDM transceiver of the present invention, the respective optical pulses constituting an optical pulse signal must be generated from light rendered by mixing light of mutually different wavelengths in at least a number equal to the number of multiplexed channels
The encoding and decoding when code set for the encoder and code set for the decoder are the same will now be described with reference to FIGS. 1A to 1D. As shown in FIG. 1A, on the transmission side 20, an optical pulse signal 9s is encoded by an encoder 10 with a function for encoding by means of code supplied by Code 1, whereby an encoded optical pulse signal 11s is generated. The encoded optical pulse signal 11s is a signal generated by performing encoding by dividing the optical pulse signal 9s into chip pulses to form an array by means of time spreading/wavelength hopping code.
That is, the encoder 10 has a function for dividing and arranging optical pulses that constitute the optical pulse signal 9s one by one into chip pulses by generating a time lag difference between wavelengths components on the time axis. When the process in which the optical pulse signal 9s is divided into chip pulses by the encoder 10 is considered, it is clear that this is a process in which one optical pulse is spread and arranged on the time axis and hopping is performed for each wavelength. As a result, encoding performed by the encoder 10 is called time spreading/wavelength hopping encoding.
An encoded optical pulse signal 11s is transmitted to the reception side 30 as a result of propagation through an optical fiber which is a transmission line 12. On the reception side 30, a playback optical pulse signal 15s that is the same as the original optical pulse signal 9s is played back as a result of decoding by a decoder 14 with a function for decoding by means of code that is supplied by Code 1. The optical pulse signal 9s is generated by intensity-modulating the optical pulse train to the RZ (Return-to-Zero) format. FIG. 1B schematically shows the optical pulse signal 9s. Further, FIG. 1C schematically shows the encoded optical pulse signal 11s. In addition, FIG. 1D schematically shows the played back playback optical pulse signal 15s. 
The same code supplied by Code 1 is set for the encoder 10 and decoder 14 shown in FIG. 1A. Hence, as shown in FIG. 1D, the playback optical pulse signal 15s that is intensity modulated to the RZ format like the optical pulse signal 9s is played back. That is, the optical pulse signal 9s is transmitted safely to the reception side 30. As shown in FIG. 1D, when encoded, the playback optical pulse signal 15s that is obtained as a result of being decoded by the same code is also known as the auto-correlation waveform of the optical pulse signal 9s. 
On the other hand, when the code of the encoder and the code of the decoder are different, the playback optical pulse signal is not obtained. How to perform the aforementioned encoding and decoding when the code of the encoder and code of the decoder are different will be described with reference to FIGS. 2A to 2D.
As shown in FIG. 2A, on the transmission side 20, the encoded optical pulse signal 11s is generated as a result of encoding by the encoder 10, which has a function for encoding the optical pulse signal 9s by means of code that is supplied by Code 1. The encoded optical pulse signal 11s is the same signal as that shown in FIG. 1C. The encoding light pulse signal 11s transmitted to the reception side 32 as a result of being propagated by the optical fiber, which is transmission line 12. On the reception side 32, the encoded optical pulse signal 11s is decoded by a decoder 16 with a function for decoding by means of code that is supplied by Code 2, whereby the playback optical pulse signal 17s is obtained. FIG. 2D schematically shows the decoded playback optical pulse signal 17s. 
Code supplied by Code 1 is set for the decoder 10 shown in FIG. 2A while code supplied by Code 2 is set for the decoder 16. Because Code 1 and Code 2 supply different code, the decoder 16 does not play back the optical pulse signal that corresponds to the original optical pulse signal 9s. A waveform from which the original optical pulse signal 9s cannot be recovered as shown in FIG. 2D is also known as a cross-correlation waveform with respect to the optical pulse signal 9s. 
In OCDM that uses time spreading/wavelength hopping, an optical pulse signal is transmitted after being encoded and multiplexed by using code that is distinct on the transmission side for each channel. Thereafter, a signal that is transmitted by multiplexing a plurality of channels is also called an optical code division multiplexed signal.
The optical code division multiplexed signal is decoded on the reception side. The waveform obtained through the decoding takes the form of the sum of an auto-correlation waveform component and a cross correlation waveform component. This is because a plurality of channels' worth of optical pulse signals that have undergone time spreading/wavelength hopping are multiplexed in an optical code division multiplexed signal. That is, this is because code that is used when encoding is performed on the transmission side and code set for the decoder is mixed in the optical code division multiplexed signal along with channels that match and channels that do not match.
The waveform that is to be extracted from the waveforms that are outputted from the decoder on the reception side is only the auto-correlation waveform component. That is, the cross correlation waveform component constituting the waveform outputted from the decoder is a noise component of the auto-correlation waveform component. One of the indices for evaluating the reception quality is the ratio of the intensities of the auto-correlation waveform component and cross correlation waveform component. That is, the greater the intensity of the auto-correlation waveform component among the waveforms outputted from the decoder, the better the reception quality. Therefore, a study of how the cross correlation waveform component can be effectively removed from the waveforms outputted from the decoder was undertaken.
Means for removing the cross correlation waveform component include a method of performing time gate processing (See “Optical Code Division Multiplexing (OCDM) and Its Applications to Photonic Networks”, Ken-ichi Kitayama, Hideyuki Sotobayashi, and Naoya Wada, IEICE Trans. Fundamentals, Vol. E82-A, No. 12 (1999), for example). Time gate processing is a method of adjusting the reception timing for each channel so that the auto-correlation waveform component and cross correlation waveform component do not overlap on the time axis after decoding. That is, this is a method for extracting only the auto-correlation waveform component by allowing the auto-correlation waveform component to pass by opening the gate for only the time required for the auto-correlation waveform component to pass after decoding and by closing the gate in the time zone in which the cross correlation waveform component passes through.
Time gate processing will be described by taking the example of a case where two-channel multiplexing transmission in OCDM using time spreading/wavelength hopping code with reference to FIGS. 3A to 3E and 4A to 4D. Further, thereafter, when reference is made to the whole of a plurality of drawings such as FIGS. 3A to 3E, for example, an unmixed range of drawings is sometimes abbreviated simply as FIG. 3. That is, when abbreviated simply as FIG. 3, this denotes FIGS. 3A to 3E.
The horizontal axis, which shows the state of an optical pulse that is shown in FIGS. 3 and 4, is the time axis that has been scaled using an optional scale and the vertical axis scales the optical intensity by means of an optional scale.
FIG. 3 serves to illustrate a description of the encoding of two-channel multiplexing OCDM. FIG. 4 serves to illustrate decoding that includes time gate processing of two-channel multiplexing OCDM. Here, for the sake of simplicity, the first channel (abbreviated as ‘Ch1’ in FIGS. 3 and 4) and second channel (abbreviated as ‘Ch2’ in FIGS. 3 and 4) are a single optical pulse. The single optical pulse is generated from light rendered by mixing light of wavelengths of four types which are λ1, λ2, λ3, and λ4.
FIGS. 3 and 4 show optical pulses as square waves as shown in FIGS. 1B to 1D and 2B to 2D. In order to illustrate the fact that this is light of the wavelengths λ1, λ2, λ3, and λ4, the codes λ1, λ2, λ3, and λ4 are appended in the rectangles representing the square waves. Because a single optical pulse is generated from light rendered by mixing light of the wavelengths λ1, λ2, λ3, and λ4, rectangles to which the codes λ1, λ2, λ3, and λ4 have been appended are shown stacked on the time axis.
The first channel in FIG. 3 will be described first. A single optical pulse of the first channel is shown in FIG. 3A. The optical pulse is generated from light rendered by mixing light of the wavelengths λ1, λ2, λ3, and λ4. The optical pulse shown in FIG. 3A is encoded by code supplied by Code 1 and, as shown in FIG. 3B, has a shape rendered through division into chip pulses and arrangement by means of time spreading/wavelength hopping on the time axis.
In keeping with the rule that the wavelengths of chip pulses should be arranged with respect to the positions in which the chip pulses exist and 0's should be arranged with respect to the positions where chip pulses do not exist, on the time axis, supposing that code is shown in the form of a progression that is lined up on one row on the time axis, the code supplied by Code 1 is written as (λ1, 0,0,0,0, λ2, 0,0,0,0, λ3, 0,0,0,0, and λ4). Thereafter, the fact that code supplied by Code 1 is shown by means of the above progression is abbreviated in the format Code 1=(λ1, 0,0,0,0, λ2, 0,0,0,0, λ3, 0,0,0,0, and λ4).
That is, it can be considered that code that is supplied by Code 1 is a function in which a single optical pulse on the time axis is converted to chip pulses that are arranged distributed along the time axis in a sequence that is provided by the above progression. Naturally, the code supplied by Code 1 is a function that plays the role of converting the respective optical pulses of a plurality of optical pulses (optical pulse signal) into chip pulses that are arranged distributed along the time axis in a sequence that is provided by the sequence supplied by the above progression.
Similarly, the second channel of FIG. 3 will now be described. A single optical pulse of the second channel is shown in FIG. 3C. As per the first channel, the optical pulse is also generated from light that is rendered by mixing light of the wavelengths λ1, λ2, λ3, and λ4. The optical pulse of the second channel that is shown in FIG. 3C is encoded by code that is supplied by Code 2 and has a shape rendered through arrangement by means of time spreading/wavelength hopping on the time axis as shown in FIG. 3D.
Similarly to the code used for the first channel and Code 1, the code used for the second channel and Code 2 are expressed in the form of a progression as follows. That is, Code 2=(0,0, λ2, 0,0,0,0,0, λ4, λ1, 0,0,0,0,0, λ3).
The result of multiplexing the first and second channels above is the arrangement shape of the channel pulses shown in FIG. 3E (encoded optical pulses). The arrangement of chip pulses shown in FIG. 3E combines the encoded optical pulses of the first channel that are encoded by means of Code 1 shown in FIG. 3B and the encoded optical pulses of the second channel that are encoded by means of Code 2 shown in FIG. 3D.
An aspect in which decoding is performed on the reception side will be described next with reference to FIG. 4. On the reception side, a chip pulse train for the optical pulses of the encoded first and second channels shown in FIG. 3E is divided into a number of channels (two here). As a result, the first channel is divided into the chip pulse train shown in FIG. 4A and the second channel is divided into the chip pulse train shown in FIG. 4C.
The chip pulse train of the first channel shown in FIG. 4A is outputted from the gate such that the gate signal for the first channel allows only the auto-correlation waveform component to pass while blocking the other chip pulses. The auto-correlation waveform of the first channel is shown in FIG. 4A such that the rectangles to which the codes λ1, λ2, λ3, and λ4 are appended are stacked vertically on the time axis. The auto-correlation waveform for the first channel that is outputted by the gate is shown in FIG. 4B.
Similarly, the chip pulse train of the second channel shown in FIG. 4C is outputted from the gate such that the gate signal for the second channel allows only the auto-correlation waveform component to pass while blocking the other chip pulses. The auto-correlation waveform of the second channel is similarly shown in FIG. 4C such that the rectangles to which the codes λ1, λ2, λ3, and λ4 are appended are stacked vertically on the time axis. The auto-correlation waveform for the second channel that is outputted by the gate is shown in FIG. 4D.
As described earlier, only the auto-correlation waveform from which the cross correlation waveform has been removed is played back by performing time gate processing on the reception side. Thus, the reception quality can be improved by performing time gate processing.
In order to implement time gate processing, it is necessary to allow the auto-correlation waveform component to pass by opening the time gate after decoding for the time taken for the auto-correlation waveform component to pass and to close the time gate in the time zone in which the cross correlation waveform passes. For this purpose, there must be synchronization between the time for opening and closing the time gate by means of any method and the time taken for the auto-correlation waveform component to pass. That is, a clock signal for acquiring synchronization must be extracted.
Therefore, a method in which an optical signal of a waveform that is rendered by combining a decoded cross correlation waveform and an auto-correlation waveform component (this optical signal is sometimes referred simply as a ‘cross correlation signal’ hereinafter) is divided into two and one part is used for extraction of the clock signal while the optical pulse signal is extracted from the other has been proposed (See “Transparent Virtual Optical Code/Wavelength Path Network”, Hideyuki Sotobayashi, Wataru Chujo, and Ken-ichi Kitayama, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 8, No. 3 (2002), for example).
However, in order to divide the correlation waveform signal into two parts by means of a branching filter and extract the clock signal from one part, it is necessary to divide the correlation waveform signal into two by means of an optical branching filter up until the cross correlation waveform is removed after decoding the optical code division multiplexed signal. Hence, the intensity of the correlation waveform signal decreases as a result of the branching loss of the optical branching filter and the insertion loss through insertion into the optical branching filter. As a result, there is the problem that the signal to noise ratio (SNR) of the played back optical pulse signal constituting the reception signal decreases. There is also the problem that of the increase in the number of parts constituting the receiver on the reception side that comprises the decoding function.